Time domain multiple access communications networks employ links such as a commonly connected, time divided bus or alternately a time shared satellite transponder, to transmit time synchronized bursts of digital data which are time multiplexed without overlapping. For example, in time domain multiple access (TDMA) satellite communications networks, a plurality of radio stations communicate through an earth satellite repeater by transmitting time synchronized bursts of radio energy relative to the repeater and which receive a time multiplexed composite of bursts containing corresponding modulated information from the repeater. In TDMA operations, multiple ground stations associated with radio signaling nodes transmit bursts of time concentrated information signals on a shared carrier frequency spectrum and receive the same information signals after repetition by the satellite repeater on a shifted carrier frequency spectrum. Each ground station is assigned a particular time slot in a continuum of recurrent frames for transmission of its bursts and for the reception of its own bursts and the bursts of other stations. The bursts interleave at the satellite in close time formation without overlapping. Each earth station includes connections to incoming digital lines originating from terrestrial sources.
An example of such a system is described in U.S. Pat. No. 4,262,356 to Lautier, et al entitled "Method and System for Synchronizing a TDMA Communication Network Comprising a Satellite Equipped with Several Directional Beam Antennas Transmitting Signals at Various Frequencies," which is assigned to the IBM Corporation. The relative position of the earth stations and the satellite transponder for the TDMA communications network is illustrated in FIG. 1. Three earth stations A, B and C on the surface of the earth 40 communicate via the synchronous satellite transponder 45 which orbits at approximately 22,500 miles above the surface of the earth 40, in a geosynchronous orbit. A transmission 42 from one of the stations B, for example, is received by the satellite transponder 45 and retransmitted over paths 42, 44 and 46 to all three of the stations A, B and C. The Lautier, et al patent describes a peer or democratic communications network where the method of synchronization allows a given station to determine the time at which its bursts are to be transmitted by detecting the time of reception of bursts originating from the other stations. In any TDMA frame, each station transmits its burst during predetermined time intervals. Each burst has the format shown in FIG. 2 herein, where a unique word (UW) is followed by the address of the transmitting station. A receiving station, for example station A detects a unique word and then decodes the transmitting station's address which follows that unique word, for example the address of station B, and initiates a time associated with station B. This timer defines the appropriate interval dT.sub.AB which must elapse between the receipt of the address from station B and a start of station A's transmission of the next burst.
In normal operation, if all stations are transmitting, three unique words (UW) and three transmitting station address are detected within one TDMA frame duration. In such a system as is described by Lautier, et al, each station is equipped with as many timers as there are stations in the TDMA network so that each timer can be associated with a different TDMA station signal which is received at a particular station. Each respective timer at a given receiving station is started at the moment that the burst from the transmitting station is received and all of the timers at a station should complete the timing of their respective intervals substantially simultaneously, that instant of termination indicating the approximate time at which that station should commence its own transmission. This can be seen with reference to FIG. 4 herein, which is a waveform diagram of bursts which are transmitted and received in a TDMA network in accordance with the Lautier, et al patent. The waveform T.sub.XA corresponds to the transmission of the burst from station A. The waveform T.sub.XB corresponds to the burst transmitted from station B. The waveform T.sub.XC corresponds to the burst from station C. Since the satellite transponder is in geosynchronous orbit, the time of reception of the transmitted bursts at any one station is delayed with respect to the time of transmission. The waveform R.sub.XA,B,C represents the waveform of the received signals which are the composite bursts from station A, station B and station C. It is seen that the burst transmitted from station A starting at time t.sub.1 is received for example at station A from the satellite transponder at the later time t.sub.2. Such delays can be as long as 300 milliseconds. For ease of explanation, the delay between the reception and transmission of a given burst will be shown as a relatively small value with respect to the duration of the bursts themselves, however it should be understood that the delay between transmission and reception can be arbitrarily long and yet the operation of the invention disclosed herein will still remain effective.
FIG. 4 also shows the relative timing of three separate timers in station A, each timer corresponding to one of the three transmitting stations A, B or C. The first timer in station A corresponding to the transmissions from station A, corresponds to the waveform labeled T.sub.AA shown in FIG. 4. At the instant t.sub.2 when the transmitted burst T.sub.XA from station A is received at station A, the timer T.sub.AA starts timing and continues for a duration dT.sub.AA until the instant t.sub.7. In a similar manner, a second timer at station A represented by the waveform T.sub.AB starts timing at the time t.sub.4 when the burst transmitted from station B at time t.sub.3 is received at station A. The timer T.sub.AB starts at the instant t.sub.4 and continues for a duration dT.sub.AB until the time t.sub.7. In a similar manner, the third timer at station A represented by the waveform T.sub.AC corresponds to bursts originating from station C and starts at the instant t.sub.6, the time of reception of the burst from station C. The timer T.sub.AC continues for a duration dT.sub.AC until the instant t.sub.7. It is noted that all three timers at station A stop at the instant t.sub.7. After a guard time T.sub.G at station A which commences at the instant t.sub.7 and ends at the instant t.sub.8, the transmission of a burst from station A commences, as is shown for the waveform T.sub.XA, and continues until the instant t.sub.8 '. As is seen in FIG. 4, the operation of the three timers at station A continues in this fashion, with the simultaneous termination of the three timers at station A signifying the starting of the guard time T.sub.G after which station A's transmission burst begins.
The operation at station B is similar to that described for station A, except that the timers T.sub.BB, T.sub.BC and T.sub.BA, corresponding respectively to bursts received from transmitting stations B, C and A, respectively, all simultaneously terminate at the instant t.sub.10. Following the guard time T.sub.G after the instant t.sub.10, the transmission burst from station B commences at the instant t.sub.11. The transmission of the burst from station A terminates before the commencement of the transmission of the burst from station B in order to insure that signals from the two respective stations will not overlap when they reach the satellite transponder 45.
The operation at station C is similar to that described for stations A and stations B, station C having three timers T.sub.CC, T.sub.CA and T.sub.CB corresponding respectively to bursts received from stations C, A and B. The three timers at station C all terminate simultaneously at the time t.sub.13, which starts the guard time T.sub.G, after which the burst from station C is transmitted at time t.sub.14. The bursts transmitted from station A and station B will have terminated prior to the commencement of the transmission of the burst T.sub.XC from station C, in order to insure that the bursts transmitted from the three stations do not overlap at the satellite transponder.